Conventional gas turbine engines are enclosed in an engine case and include a compressor, combustor, and a turbine. Air flows axially through the sections of the engine. As is well known in the art, air, compressed in the compressor, is mixed with fuel which is burned in the combustor and expanded in the turbine, thereby rotating the turbine and driving the compressor.
Most compressors include a fan, a low pressure compressor, and a high pressure compressor disposed about a longitudinal axis of the engine with the low pressure compressor rotating at lower speeds than the high pressure compressor. Both, the low pressure compressor and the high pressure compressor, as well as the turbine, comprise alternating stages of rotating airfoils, or blades, and stationary airfoils, or vanes.
The gas turbine engines mounted on a wing of an aircraft are frequently damaged by foreign objects, such as sand particles or stones, ingested by the engine during takeoff. The foreign objects ingested by the engine often cause nicks or chips on impact with the compressor airfoils. Most damage caused by the foreign objects is to the first few stages of the high pressure compressor blades. Furthermore, most damage occurs at the leading edge of each affected blade.
It is necessary to detect damage and then replace or repair blades when damage exceeds industry acceptable limits. The detection process involves a visual inspection of each blade through a borescope. The borescope, a fiber optic cable connected to a light source, is inserted through borescope openings within the engine case and into the engine. The small borescope openings are disposed throughout the engine case at most stages of the high pressure compressor for such borescope inspections.
In accordance with certain prior art inspection techniques, once a blade having excessive damage is observed, the engine must be removed from the wing of the aircraft. Subsequently, the engine has to be disassembled to expose the damaged blade. Only then, can the blade be accessed and either repaired or replaced. This procedure is time consuming and extremely expensive. The cost of removing, disassembling, repairing and reassembling the engine is between $250,000 and $500,000.
The industry has found a more practical solution to repair gas turbine engine airfoils. Machida, Inc. of Orangeburg, N.Y. sells a Power Blending Borescope Kit for JT8D-200 engine, manufactured by the Pratt & Whitney Division of United Technologies Corporation of Hartford, Conn., the assignee of the present invention. The Power Blending Borescope Kit, developed jointly by Pratt & Whitney and Machida, Inc., detects and repairs (i.e. blends) foreign object damage on the seventh stage compressor blades of JT8D-200 series engine. The kit includes a grinding tip connected to a drive cable that passes through a working channel of a flexible borescope. The working channel is flexible to the extent that it is capable of being bent by a wire. After the flexible working channel in a straight position is inserted into the engine through the seventh stage borescope opening, a knob is turned to activate the wire to bend the flexible channel for accessing the leading edge of the seventh stage blade at a substantially right angle. The grinding tip must be positioned at a substantially right angle to the leading edge of the blade during the blending process so that the blended edge of the airfoil is blunt. If the grinding tip accesses the leading edge at an angle other than a substantially right, the resulting blended edge will be a sharp edge, which is unacceptable for operation of the gas turbine engine. Subsequent to proper positioning of the grinding tip, a drive motor, connected to the drive cable, operates the grinding tip to blend the damaged area of the blade. A technician operating the kit views the damaged airfoil and the process of repairing the airfoil on a monitor.
Although this system eliminates the need to remove and disassemble the engine, it has a number of limitations. One major drawback is that this blending tool can access only certain stages of certain engine models. The first underlying reason for this blending tool not being capable of accessing the leading edge of the blade is that newer engines have more intricate passageways between the borescope opening and the leading edge of the blade. In order to reach the leading edge of the blades of certain stages, the flexible channel must have multiple bends. The current construction cannot have more than one bend and has limitations on certain angles since the bending motion of the flexible working channel is motor driven and bending in more than one direction is not possible. The second underlying reason for this blending tool not being capable of accessing the leading edge of the blade is that on some newer engines, such as the PW4084 manufactured by Pratt & Whitney, different stages of the high pressure compressor have varying distances between the borescope openings and the blades. The existing tool cannot be adapted to have a retractable and extendible grinding tip to compensate for the varying length of the passageways of the newer engines.
Another shortcoming of the above described tool is that once the grinding tip comes into contact with a sharp burr, the tool tends to vibrate and travel away from the damaged area. The flexible channel cannot transmit enough force from the operator to maintain the grinding tip in close, accurate contact with the damaged area of the blade.
A further drawback of the above described tool is that the grinding tip is removable. After the grinding is completed, a polishing tip replaces the grinding tip. If the grinding tip is not tightened sufficiently and falls into the engine, the engine may have to be removed from the wing and disassembled to retrieve the grinding tip, a complicated and, as noted above, an expensive procedure. Thus, there is still a need for a blending tool for modern gas turbine engines that does not require removal of the engines from the airplane and allows proper access to the high pressure compressor blades at all necessary stages thereof.